As described by F. Capasso et al. in Solid State Communications, Vol. 102, No. 2-3, pp. 231-236 (1997) and by J. Faist et al. in Science, Vol. 264, pp. 553-556 (1994), which are incorporated herein by reference, a QC laser is based on intersubband transitions between excited states of coupled quantum wells and on resonant tunneling as the pumping mechanism. Unlike all other semiconductor lasers (e.g., diode lasers), the wavelength of the lasing emission of a QC laser is essentially determined by quantum confinement; i.e., by the thickness of the layers of the active region rather than by the bandgap of the active region material. As such it can be tailored over a very wide range using the same semiconductor material. For example, QC lasers with AlInAs/GaInAs active regions have operated at mid-infrared wavelengths in the 3 to 13 .mu.m range. In diode lasers, in contrast, the bandgap of the active region is the main factor in determining the lasing wavelength. Thus, to obtain lasing operation at comparable infrared wavelengths the prior art has largely resorted to the more temperature sensitive and more difficult-to-process lead salt materials system.
More specifically, diode lasers, including quantum well lasers, rely on transitions between energy bands in which conduction band electrons and valence band holes, injected into the active region through a forward-biased p-n junction, radiatively recombine across the bandgap. Thus, as noted above, the bandgap essentially determines the lasing wavelength. In contrast, the QC laser relies on only one type of carrier; i.e., it is a unipolar semiconductor laser in which distinct electronic transitions between conduction band states occur; these states arise from size quantization in the active region heterostructure.
The active region of QC lasers includes a multiplicity N (typically .about.25) of stacked stages or repeat units, each unit including a radiative transition (RT) region adjacent an injection/relaxation (I/R) region. As the name of the laser implies, electrons cascade from one RT region to the next when a suitable bias voltage is applied across the device. This scheme leads to a slope efficiency (and external differential quantum efficiency) and laser power proportional to N in the linear part of the L-I characteristic, as confirmed by J. Faist et al., IEEE J. Quantum Electron., Vol. 34, No. 2, pp. 336-343 (Feb. 14, 1998) and C. Gmachl et al., Appl. Phys. Lett., Vol. 72, No. 24, pp.3130-3132 (Jun. 15, 1998), both of which are incorporated herein by reference. The N stages, which form a common active waveguide core, also increase the optical confinement and, therefore, the modal gain sufficiently to overcome the increased waveguide losses at mid-infrared (e.g., 3-13 .mu.m) wavelengths, especially when N is large (e.g., .gtoreq.10). In fact, early theoretical work of Yee et al., Appl. Phys. Lett., Vol. 63, No. 8, pp. 1089-1091 (1993) on GaAs/AlGaAs lasers reached the conclusion that a single intersubband transition (in a device having only a single RT region) would not have enough gain in the mid-infrared to reach the lasing threshold. Indeed, the high performance of many QC lasers has heretofore reinforced the notion that a cascaded structure is essential for an intersubband laser.
Thus, a need remains for a unipolar, intersubband semiconductor light source, especially a laser, that is capable of efficient emission at a single intersubband transition in a device having a single RT region.
In addition, the tendency for prior art QC lasers to have a relatively large number of stages (e.g., 25, as noted above) has both processing and operational implications. From a fabrication standpoint a large number of stages means a corresponding large number of relatively thin layers (e.g., 400) have to be epitaxially grown, which in turn requires careful control of the growth process (as to layer thickness, composition and strain compensation). On the other hand, from an operational perspective, a large number of stages typically requires a higher voltage bias (e.g., 6-10 V). Yet, some applications require an optical source which is capable of operation at lower voltages (e.g., &lt;3 V).
Therefore, there is also a need for a unipolar, intersubband semiconductor light source that operates at relatively low voltages.
There is likewise a need for such a light source that has fewer layers and, therefore, is simpler to fabricate.